The present invention relates to a thermal printer in which density gradation of each picture element is controlled by causing a current to flow into each heating resistor element for a current conduction time of an integer multiple of a current conduction unit time, and particularly relates to a thermal printer in which a power supply circuit for driving heating resistor elements has its load reduced, or has its arrangement simplified and in which the recording quality is made stable.
In such a thermal printer, the current conduction time for each heating resistor element of a thermal head is controlled to thereby make the heating energy of the heating resistor element have gradation so as to give gradation to the transfer quantity of ink and hence to the density of the picture.
FIG. 1 shows a current-conduction-time-to-density characteristic curve used in a density gradation system. If a current is made to flow in a heating resistor element for a current conduction time T.sub.1 corresponding to a current conduction unit time .DELTA.T, the density of a gradation level d.sub.1 is obtained, and if a current is made to flow in the heating resistor element for a current conduction time T.sub.2 corresponding to double of the current conduction unit time .DELTA.T, the density of a gradation level d.sub.2 is obtained. In this example, if a current is made to flow in the heating resistor element for a current conduction time T.sub.64 corresponding to 64 times the current conduction unit time .DELTA.T, the density of a maximum gradation level d.sub.64 near saturation density is obtained.
FIG. 2 schematically shows a main arrangement of a conventional density gradation control type thermal printer. A thermal print head 100 is provided with a heating resistor 102 constituted by an array of a plurality (for example, 512 pieces) of heating resistor elements R.sub.1 to R.sub.512, a shift register 104 and a latch circuit 106, each of the shift register 104 and the latch circuit 106 having a capacity of bits equal in number (512) to the number of resistor elements. A gradation data supply section 108 supplied with 512 bits of serial gradation data [C.sub.K P.sub.1j -C.sub.K P.sub.512j ] supplies the gradation to the shift register 104 plural times, for example, 64 times (K=1-64), successively at regular intervals. Here, the gradation data C.sub.K P.sub.nj of the n-th bit includes information as to whether a current is to be made to flow into the n-th heating resistor element R.sub.n for the current conduction unit time .DELTA.T or not. Specifically, if the information is "1", it indicates current conduction, and if "0", it indicates non-conduction.
Then, if the gradation data C.sub.K P.sub.1j to C.sub.K P.sub.512j in each time have been loaded into the shift register 104 in synchronism with a clock signal CK, the gradation data C.sub.K P.sub.1j to C.sub.K P.sub.512j at the respective bits of the shift register 104 are fed in the form of electric pulses to the heating resistor 102 through the latch circuit 106, so that currents are made to flow selectively into the heating resistor elements R.sub.1 to R.sub.512 for the current conduction unit time .DELTA.T in accordance with the contents of information of the corresponding bits so that the heating resistor elements R.sub.1 to R.sub.512 generate heat selectively.
Such an operation is repeatedly carried out 64 times (K=1-64) per print line in accordance with the gradation data in a period (in a conduction mode) during which a strobe signal ST is generated, so that the current conduction for the current conduction time T.sub.64 corresponding to 64 times the current conduction unit time .DELTA.T is performed at its highest limit to thereby make it possible to give any one of 64 density gradation levels to each of the picture elements on each print line.
That is, corresponding to the number of times of the gradation data during which the gradation bit C.sub.K P.sub.nj continuously has its information contents being "1", the number of times of current conduction to the corresponding heating resistor element R.sub.n is determined, so that the gradation level of density of the corresponding picture element is determined. For example, assume that the gradation bit C.sub.K P.sub.1j for the heating resistor element R.sub.1 continues being a "1" until the tenth gradation data, then the bits C.sub.1 P.sub.1j to C.sub.10 P.sub.1j become "1" respectively and the bits C.sub.11 P.sub.1j to C.sub.64 P.sub.1j become "0" respectively, so that a current is made to flow into the heating resistor element R.sub.1 for a current conduction time T.sub.10 corresponding to ten times the current conduction unit time .DELTA.T, and the density gradation level of the corresponding picture element becomes d.sub.10. If the gradation bit C.sub.K P.sub.2j for the the heating resistor element R.sub.2 continues being a "1" until the seventh gradation data, then the bits C.sub.1 P.sub.2j to C.sub.7 P.sub.2j become "1" respectively and the bits C.sub.8 P.sub.2j to C.sub.64 P.sub.2j become "0" respectively, so that a current is made to flow into the heating resistor element R.sub.2 for a current conduction time T.sub.7 corresponding to seven times the current conduction unit time .DELTA.T and the density gradation level of the corresponding picture element becomes d.sub.7 (referring to FIG. 1).
As described above, conventionally, if the current conduction mode is started, all the heating resistor elements corresponding to the picture elements to be recorded start current conduction simultaneously with each other, and the respective picture elements are given their density gradation corresponding to the length of time from the initiation of the current conduction to the point of time when the current conduction of the corresponding heating resistor elements terminates.
However, since the heating resistor elements start current conduction simultaneously with each other as soon as the current conduction mode is started, the sum of the currents flowing into the whole of the heating resistor elements increases suddenly, so that the driving voltage applied to the respective heating resistor elements drops down suddenly due to the steep current surges.
FIG. 3 shows the above status. In a period TE (the current conduction mode) in which the strobe signal ST (FIG. 3A) is "0", the heating resistor elements R.sub.1 to R.sub.512 are subject to current conduction for the time corresponding to the density gradation of the corresponding picture elements to be recorded. For example, on a print line L.sub.N, the heating resistor element R.sub.1 (FIG. 3B) is subject to current conduction until the maximum time T.sub.64 to thereby record a picture element with the maximum density gradation d.sub.64, and the heating resistor element R.sub.2 (FIG. 3C) is subject to current conduction for the time T.sub.34 to thereby record a picture element with an almost medium density gradation d.sub.34. A period TC in which the strobe signal ST is "1" is a cooling mode. The period TC of the cooling mode is selected to be a time having the length as long as the necessary minimum for cooling the heating resistor elements heated by current conduction for the maximum permitted current conduction time (T.sub.64).
In printing of a print line L.sub.N or L.sub.N+1, immediately after the starting point of time t.sub.N or t.sub.N+1 in the current conduction mode TE, all the heating resistor elements R.sub.1, R.sub.2, . . . , R.sub.512 except the heating resistor elements not to record picture elements are subject to current conduction at the same time, so that the current I supplied to the whole of the heating resistor elements (hereinafter referred to as a whole conduction current) increases suddenly (FIG. 3E). Then, the voltage drop in the conductors, such as cables, printed wires, and so on connecting a power supply circuit to the heating resistor elements rises suddenly, so that the driving voltage V actually applied to the heating resistor elements drops down largely (FIG. 3F). After this voltage drop DR, the driving voltage V is made to come back to a reference value V.sub.F by the operation of voltage compensation. That is, used is a voltage compensating circuit in which the driving voltage V is detected and fed back to the power supply circuit so as to keep the driving voltage V constant.
However, if the response of the voltage compensating circuit is low, it takes some time for the recovery of the driving voltage V, so that it is necessary to make a design to prolong the current conduction time of the heating resistor elements correspondingly for the sake of the guarantee of recording quality. This is not preferable in that not only the adjustment is difficult, but also that time required for printing is prolonged.
If a voltage compensating circuit of the high speed response type is used, the apparatus is considerably expensive and the above-mentioned problem cannot be fundamentally solved correspondingly to the cost.